Research communities and microelectronics industries have for some time known about and used micro-fabricated charge-emission devices. Two types of charge-emission devices are field emission devices, which emit electrons, and field ionization devices, which emit ions. One class of charge-emission devices, referred to as a gated charge-emission device, has a gate electrode in close proximity to the tip of one or more emitters. In general, a voltage applied to the gate electrode relative to the tips of the emitters controls the quantity of charge emitted by the charge-emission device. Once the voltage exceeds a threshold, which can vary among the emitters, the charge-emission device begins to emit charge. A further increase in voltage induces a corresponding increase in the emitted charge. When the voltage falls below the threshold, the emitters cease to emit charge.
Because of the small scale of geometries of the gate electrode and emitters, micro-fabricated charge-emission devices require relatively low power to emit charge efficiently. Typically, the operating voltage for inducing charge emission from an emitter tip ranges between 50 and 100 volts. Consequently, micro-fabricated charge-emission devices are being used in a variety of applications, such as ion thrusters, micro-fluidic dispensers, and satellite charge controllers.
Notwithstanding their emission efficiency, charge-emission devices can be unstable as current sources. Fluctuations in the amount of emitted charge are highly dependent on the surface physics at each emitter tip and on the equilibrium of that emitter tip with its environment. Consequently, the amount of emitted charge can be difficult to control and susceptible to instabilities.
A typical technique to control charge emission is to construct a feedback system around the charge-emission device. In a typical feedback system, an adjustable voltage supply applies a voltage across the gate electrode and the emitters to induce the emitters to emit charge. A meter then measures the flow of charge through the device and, if the current measurement indicates that the flow of charge is not at a desired level, the applied voltage is adjusted accordingly. The process may repeat until the feedback system achieves the desired current emission level. Often the responsiveness of the feedback system is slow, inefficient, inaccurate, and susceptible to the variability of the emitters. Further, if the charge-emission device enters a runaway emission condition, the feedback system operates too slowly to avoid irreparable damage to the device.
Thus, there remains a need for a system and method for controlling charge emission by a charge-emission device that avoid the aforementioned disadvantages.